feat: generate alt text in charts

[alexdln]

Alt text may be important for my users and systems. It's needed both for immediate access to content and for sharing later in an A11Y-friendly environment (like bluesky).

Content needs to be generated based on the chart. Localization is possible if we can localize the entire chart as well.

It's usually standard practice to add the ability to copy alt text in the options.

Services typically have a character limit of 1000-2000. Bluesky has 2000 characters, so I think it's worth sticking to it.

[graphieros]

Version 3.15.0 of vue-data-ui is now ready for this feature to be discussed and implemented.
See #1455

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Alt text copy new action button implementation on chart configurations

• In the charts context menu, a new 'Copy alt text' button can be enabled
• Paired with a callback, that exposes the chart's dataset and config, this action can be used to tailor an alt text and copy it to the clipboard.

const config = computed(() => (
  userOptions: {
    buttons: {
      altCopy: true, // false by default. Set to true to display the button in the context menu
    },
    callbacks: {
      altCopy: ({ dataset: dst, config: cfg }) => {
        // Convert the data into a tailor-made alt text you can copy to the clipboard
      }
    },
    buttonTitles: {
      altCopy: 'Copy alt text' // provide your translation for the button's tooltip
    }
  }
));

Customise the icon for the context menu button, using the #optionAltCopy slot inside the chart component:

<template #optionAltCopy>
  <span
    class="i-carbon:accessibility-alt w-6 h-6 text-fg-subtle"
    style="pointer-events: none"
    aria-hidden="true"
  />
</template>

A copy validation could be displayed in there too, using a state variable